Chemical sensors have been developed in recent years for the in vivo monitoring of the concentration of various analytes, including oxygen, carbon dioxide, and hydrogen ions (i.e., pH), in liquids and gases. In particular, fiber-optic chemical sensors have been developed for measuring the levels of carbon dioxide and oxygen gases in the blood, and the pH of the blood. Such sensors typically include one or more optical fibers, on the distal end of which are mounted analyte sensors. These sensors exhibit a perturbed light absorbance, luminescence, or phosphorescence in the presence of the analyte molecules. A thermocouple may be included with these analyte sensors, the combination being configured as a probe that is insertable through a catheter into a patient's vascular system.
A saline flush solution that may contain an anti-clotting agent is often introduced through the inserted catheter at a low rate, such as 3 to 5 ccs per hour, to prevent the development of thrombi around the probe. Signals affected by the analyte sensors mounted at the distal end of the probe are periodically monitored to determine the patient's blood gas levels and temperature of the blood.
Sufficient blood flow past the probe is required to clear this saline flush solution from the vicinity of the sensors and enable a measurement of pH and the partial pressure of carbon dioxide (pCO.sub.2) and oxygen (pO.sub.2) that is accurately reflective of these analyte concentrations in the bloodstream. A problem with the accuracy of blood chemistry measurements may be experienced when certain diseases or physiological states cause a reduction in the amount of blood flow through the monitored blood vessel. Causal factors of slow peripheral blood flow include thoracic surgical procedures, cold extremities, left-side heart failure, arterial sclerosis, local vasoconstriction due to catheterization, and occlusion of the blood vessel by the catheter through which the probe is inserted.
When blood flow past the probe is reduced, the saline flush solution is not fully cleared away from the chemical sensors by the blood flow, and the volume of blood surrounding the chemical sensors is diluted. Consequently, the chemical sensors in the probe measure pH, pCO.sub.2, pO.sub.2, and temperature of a mixture of blood and saline flush, rather than systemic blood. During such periods of "flush interference" (i.e., blood mixed with substantial quantities of saline flush), the chemical sensors provide measurements that are not clinically relevent. Saline flush solution typically is characterized by pH and pCO.sub.2 values that are substantially lower than the corresponding values of systemic blood, and its pO.sub.2 may vary greatly from that of systemic blood value at any given time. Erroneous readings caused by flush interference may, at best, cause medical personnel to disregard data from the chemical sensors. When data from the probe cannot be relied upon, the practitioner is often required to draw and analyze a discrete blood gas sample from the patient. Potentially more harmful, medical personnel may instead not recognize the flush interference condition and determine patient treatment based upon the erroneous results.
Clinical and animal trails have shown that peripheral arterial blood flow compromises saline flush solution approximately 5 to 30% of the time during in vivo measurements of blood gas parameters. The prior methodology for recognizing that flush interference is occurring has been to watch for extreme drops in both the measured pH (such as a measured pH less than 7.0) and pCO.sub.2 (such as a measured pCO.sub.2 below 10 torr). Such low measurements are indicative of chronic flush interference, i.e., dilution of blood with substantial quantities of flush solution for the period of time during which the blood gas parameters are measured. Because there are few, slow onset disease states, such as chronic metabolic acidosis, that can cause this combination of low arterial pH and pCO.sub.2, diagnosis of chronic flush interference is relatively easy. However, in cases of intermittent or less extreme flush interference, the resulting discrepancies in measured pH and pCO.sub.2, as well as pO.sub.2 and temperature, are not as readily identified. In cases of significant but non-chronic flush interference, erroneous readings may either go undetected, or if detected, may result in the need for discrete blood sample analysis, with ensuring cost, delay, and potential patient discomfort.